Mycoplasma pulmonis is one of the most important pathogens of laboratory rodents. It is the etiologic agent of Murine Chronic Respiratory Disease (CRD) which was ubiquitous in laboratory rats and mice into the 1970's. This disease has almost been eliminated. In research colonies, detection of subclinical infections by parasitic, bacterial, viral, and mycoplasmal agents generally depends on testing of designated sentinel animals. Because immunodeficient mice do not make antibodies, serologic tests, which would detect them, are of no value in these animals. However, the nearly universal practice of housing immunodeficient mice in microisolator cages effectively isolates the sentinels (who may be housed in the same rack but in different cages) from any exposure to pathogens harbored by the immunodeficient mice. The objective of this study is to 1) determine the gross and histopathologic changes produced by Mycoplasma pulmonis in immunodeficient mice (SCID) inoculated by both the respiratory and genital routes and compare them with those found in immunocompetent mice (BALB/C) under identical conditions; and 2) establish the reliability of polymerase chain reaction (PCR) techniques in detecting mycoplasmosis, and compare its sensitivity with that of routine culture. Mice will be infected by either the intranasal or intravaginal route. Infection will be detected by serology (as appropriate), culture, PCR, and by gross and histologic changes. Samples for mycoplasma culture and PCR will be collected from the oropharynx, trachea, lung, and uterus. The following tissues will be screened for histologic changes: nasal sinuses, middle ears, trachea, lungs, vagina and uterus.